1. Field of the Invention
The present invention relates to AC transmission systems that use existing AC transmission lines, but are unaffected by the reactance of the transmission lines, and more particularly relates to AC transmission systems that improve the transmission capability of transmission lines or distribution lines by cancelling the impedance of AC transmission lines or AC distribution lines.
2. Description of the Related Art
Most Japanese transmission lines are AC transmission type of frequency 50 Hz or 60 Hz, while some, such as intersystem links and long-distance transmission lines, use DC transmission.
FIG. 1 shows system block diagrams illustrating the concepts of DC transmission and AC transmission.
In the diagrams, G indicates a power station, SS a sub-station, TR1, TR2 transformers, CNV an AC/DC power converter, INV a DC/AC power converter, Ld a DC reactor and X AC transmission line reactance, respectively.
FIG. 1(a) shows a DC transmission system. AC power generated in power station G is converted to DC by AC/DC power converter CNV. That DC power is transmitted to the receiving sub-station via DC transmission lines. Then, the DC power is once more converted to AC power by DC/AC power converter INV, and AC power is supplied to other sub-stations or to demand loads.
This DC transmission system has the advantage that it is possible to transmit power over long distances without voltage drops due to transmission line reactance because the power is transmitted by temporarily converting it to DC. Also, even if two AC systems have different frequencies, it is possible to link them. However, power converters each having the same transmission capacity are required on the transmitting side and the receiving side. Also, new DC transmission lines must be constructed, and the system becomes expensive.
On the other hand, FIG. 1(b) shows an AC transmission system. This has the advantage that transformers TR1 and TR2 can freely alter the voltage, and the AC can be transmitted as it is. Also, most of the present transmission lines are AC transmission lines, and an economical power supply that uses these effectively is possible. However, when there is reactance X in a transmission line, its voltage drop becomes a problem, and there is the disadvantage that there is a limit to the active power than can be transmitted.
In FIG. 2, FIG. 2(a) shows a simple equivalent circuit for an AC transmission line, and FIG. 2(b) shows the relationship of the transmitted power to phase angle .theta.. In the drawings, when the transmitting terminal voltage is taken as Vs, the receiving terminal voltage as Vr, its phase difference as .theta. and the reactance of the transmission line as X, the active power P that can be transmitted is expressed by the following expression. ##EQU1##
Consequently, as the length of the transmission line becomes longer, reactance X becomes greater, and the transmissible active power is more limited.
Also, normal transmission is operated with a phase difference .theta. of 30.degree. or less. However, if power fluctuations occur in the system and the phase difference .theta. exceeds 90.degree., a so-called generator out-of-step phenomenon occurs, and transmission becomes impossible. Moreover, in a system with a large transmission line reactance X, there is the disadvantage that, since the maximum value of transmitted power is small, phase difference .theta. will fluctuate greatly, even with a small power fluctuation, and the system readily becomes unstable.
With AC transmission, the voltage can freely be varied by a transformer, and this is economical, providing AC transmission over long distances using existing lines is possible. Thus, there is a great advantage on both the power supply side and the power user side. However, it suffers from the effect of transmission line reactance, and there are limits to the transmissible active power.
On the other hand, DC transmission does not suffer the effect of transmission line reactance. However, since it cannot use existing transmission lines, and also large capacity power converters sufficient for the transmission capacity are required, the system cost becomes very high. Moreover, when attempting to supply power to a load partway along the transmission route, it is necessary temporarily to convert the DC to AC power; power converters are required for that, and that, too, becomes an uneconomic system.